Radioactive krypton gas separation

ABSTRACT

Radioactive krypton is separated from a gas mixture comprising nitrogen and traces of carbon dioxide and radioactive krypton by first selective adsorption and then cryogenic distillation of the prepurified gas against nitrogen liquid to produce krypton bottoms concentrate liquid, using the nitrogen gas from the distillation for two step purging of the adsorbent.

This is a division of application Ser. No. 252,219, filed May 11, 1972,now U.S. Pat. No. 3,944,646.

BACKGROUND OF THE INVENTION

This invention relates to a method of and apparatus for separation ofradioactive krypton from a feed gas mixture comprising nitrogen andtrace amounts of carbon dioxide and radioactive krypton by firstselective adsorption and then cyrogenic distillation to produce highlyconcentrated radioactive gas and permit release of the remaining gas tothe atmosphere without radioactive contamination.

In the normal operation of nuclear power plants of the boiling waterreactor type, an off-gas stream is produced which is radioactive. Suchstreams threaten the environment primarily due to their krypton andxenon content, other significant radioactive components being shorthalf-life isotopes which can be satisfactorily deactivated by storageprior to being released to the atmosphere. More specifically, thenuclear reactor off-gas contains radioactive isotopes of other gaseouselements such as oxygen, nitrogen and argon. The half-lives of theseradioactive components vary widely from a fraction of a minute to about10 minutes. For example, if a value of 1.0 were assigned to theradioactivity level of a quantity of off-gas at the moment it iswithdrawn from the conventional vacuum condenser, its radioactive levelwould drop to about 1/90 and 1/550 after time lapses of 2 minutes and 30minutes respectively. However, the residual radioactivity level after 30minutes is still far greater than can be safely permitted if the gas isto be released to the atmosphere.

The discharge of radioactive off-gas creates both a short term and along term problem, the former being the total radioactivity of the gasdischarged to the atmosphere. Because the total radioactivity isproduced largely by short half-life isotopes, the short term problem islocalized within the immediate vicinity of the nuclear reactor where theradioactivity is not yet dispersed and decayed to negligble levels. Thelong term problem is concerned primarily with krypton-85 which has ahalf-life of greater than 10 years. Krypton-85 should be substantiallyremoved from the off-gas to minimize the long term accumulation ofradioactive contaminants throughout the earth's atmosphere.

In some instances, nuclear power plants have merely diluted the off-gaswith additional volumes of air before venting to the atmosphere but thisis no longer desirable. The only safe method to dispose the radioactivegas is to store it with adequate shielding for a time sufficient for theradioactivity to decay to prescribed tolerable levels, but the mostrecent stringent standards may require many weeks storage. The quantityof off-gas produced in a boiling water type nuclear power plant is quitelarge. For example, in a 1100 megawatt plant, the non-condensablescomprising the off-gas may accumulate at the rate of 200-300 cfm. (STP),so that if this gas is held in several week's "decay-storage" the amountbeing retained in any given moment can readily be several million cubicfeet (STP) requiring extremely large storage tanks which must beshielded to contain its radioactive emission.

One prior art approach has been to remove a part of the non-radioactiveconstituents of the off-gas so as to concentrate the radioactive portionto perhaps 25% of the original volume. This approach does not mitigatethe long term storage decay problem created by krypton-85. Other nuclearpower plants have delayed the release of the radioactive off-gas for ashort period, e.g. one-half hour, by passing same through an extendedpipe line system preceding the vent point. This system has been furtherimproved at certain locations by inserting a large carbon adsorptiondelay trap in the extended pipe line system preceding the vent point. Nomeans have been provided for cyclic adsorption and desorption of theadsorption trap, but by continuous adsorption displacement it retainsthe radioactive components for a longer period of time than obtainablein the pipe line alone and permits further decay of their radioactivity.In this manner, the krypton retention may be increased to several daysbut even this improvement fails to meet the increasing stringentlimitations on total radioactivity level established by governmentagencies in many locations.

It is an object of this invention to provide an improved method of andapparatus for separating radioactive krypton from a gas mixturecomprising nitrogen and trace amounts of carbon dioxide and theradioactive krytpon.

Another object is to provide an improved method of and apparatus forradioactive krypton removal from nuclear reactor off-gas so that theremaining gas may be released to the atmosphere without radioactivecontamination.

Still another object is to provide such a system requiring onlyrelatively small volume for storage-radioactive decay of the separatedkrypton.

Other objects and advantages of this invention will be apparent from theensuing disclosure and claims.

SUMMARY

This invention relates to a method of and apparatus for separation ofradioactive krypton from a gas mixture comprising nitrogen and traceamounts of carbon dioxide by first selective adsorption and thencryogenic distillation to produce highly concentrated radioactive gas.Although the invention will be broadly described in terms of removingradioactive krypton from a feed gas mixture, radioactive xenon issimultaneously removed if present and in concentrated form with the moredifficultly removed krypton. Moreover, if the feed gas mixture alsoincludes water traces, they will be removed with the carbon dioxide.

A method aspect of this invention includes providing the feed gasmixture at superatmospheric pressure and ambient temperature and passingsame through the first of at least two crystalline zeolite molecularsieve adsorption zones for preferential adsorption of carbon dioxide andcoadsorption of a minor part of the krypton. The nonadsorbed prepurifiedgas from this first adsorption zone is cooled to cryogenic temperatureand distilled against nitrogen liquid to produce a kyrpton bottomsconcentrate liquid and purified nitrogen overhead gas. The lastmentioned gas is partially rewarmed to about ambient temperature by heatexchange with the nonadsorbed prepurified gas for the aforementionedcooling of same. A first part of the partially rewarmed purifiednitrogen gas from the distillation is passed as cool purge gas at lowpressure through a second adsorption zone having previously been atleast partially loaded with carbon dioxide and krypton by passage of thefeed gas mixture therethrough. In this step, only the krypton iscompletely desorbed from the other zone (along with an unavoidable minorfraction of the carbon dioxide adsorbate).

The cool purge gas is discharged from the second zone with thisradioactive desorbate and returned with the feed gas mixture for passagethrough the first zone. A second part of the partially rewarmed purifiednitrogen gas from the distillation is further warmed to at least 350° F.and passed as hot purge gas through an other-than-first adsorption zonewhich has previously been at least partially loaded with carbon dioxideand coadsorbed krypton by passage of the feed gas mixture therethrough,only the krypton having been thereafter completely desorbed from theother-than-first adsorption zone by passage of the purge gastherethrough at about ambient temperature. The carbon dioxide is therebydesorbed (i.e. the quantity of CO₂ undergoing mass transfer during acomplete adsorption-desorption cycle) from the other-than-first zone anddischarged therefrom in the hot purge gas. A minor part of the partiallyrewarmed purified nitrogen overhead gas is passed into the feed zoneafter the hot puge gas flow for recooling of such zone.

When only two adsorption zones are used, the rewarmed purified nitrogengas need not be continuously divided into at least the aforementiondfirst and second parts because both are sequentially passed to thesecond zone for respectively cool and hot purging thereof. When threeadsorption zones are employed, the rewarmed purified nitrogen gas iscontinuously divided into at least a first part and a second part, theformer being diverted to the second zone for cool purging and the secondpart being diverted to the third zone for hot purging step.

An apparatus aspect of the invention includes at least two crystallinezeolite adsorption beds arranged in parallel flow relation, and meansfor providing the feed gas mixture at superatmospheric pressure andambient temperature and sequentially introducing same to the inlet endof each of the adsorbent beds. The apparatus also includes heatexchanger means having first and second passageways and means forpassing nonadsorbed prepurified gas from the discharge end of eachadsorbent bed to the first heat exchanger passageway for cryogeniccooling therein. The distillation column has a top reflux condenser, abottom kettle with heating means, and a multiplicity of spacedliquid-gas contact trays intermediate the top reflux condenser andbottom kettle. A liquid nitrogen supply is provided with means forintroducing same to the top reflux condenser.

The apparatus further includes conduit means for introducing thecryogenic cooled prepurified gas from the heat exchanger means to anintermediate tray section of the distillation column for mass and heatexchange with krypton-depleted condensate to form krypton-depleted vaporand krypton-enriched liquid. Conduit means pass the krypton-depletedvapor from the upper end of the intermediate tray section to the topreflux condenser for heat exchange with the liquid nitrogen supply toform nitrogen overhead gas and krypton-depleted condensate. Other meansreturn at least part of the krypton-depleted condensate to the upper endof the intermediate tray section.

Conduit means pass the nitrogen overhead gas to the second passageway ofthe aforementioned heat exchanger means for partially rewarming same toabout ambient temperature and for previously described cryogenic coolingof the prepurified gas. Other conduit and flow control means areincluded for sequentially passing a first part of the partially rewarmednitrogen overhead gas at low pressure as cool purge gas to the feeddischarge end of each adsorbent bed having previously been at leastpartially loaded with carbon dioxide and krypton from the feed gasmixture, for substantially complete desorption of only the krypton.Different conduit means are included for returning thekrypton-containing first part of cool purge gas from the adsobent bedinlet end to the aforementioned feed gas mixture providing means.

The apparatus also contemplates means for further warming a second partof the partially rewarmed nitrogen overhead gas as hot purge gas, andstill another conduit and flow control means for sequentially passingsame to the feed discharge end of each adsorbent bed having previouslybeen at least partially loaded with carbon dioxide and krypton from thegas mixture and thereafter partially desorbed of carbon dioxide andsubstantially completely desorbed of only the krypton, thereby desorbingthe remaining carbon dioxide (i.e. the remaining CO₂ undergoing masstransfer during a complete adsorption-desorption cycle). Conduit meansdischarge the carbon dioxide-containing hot purge gas from the adsorbentbed feed inlet end, and different conduit and flow control means areprovided for sequentially introducing a third part of the partiallyrewarmed N₂ overhead gas to the discharge end of each adsorbent bedafter carbon dioxide desorption for recooling the bed.

The system of this invention reduces the level of radioactivity of theoff-gas by a factor of 10⁶ compared to its level at the vacuum condenser(described hereinafter), and by a factor of 100 compared to the levelobtained in long-residence carbon traps used heretofore. The radioactivecontaminants, krypton and xenon, are 99.9⁺ % removed from the purifiedvent gas and may be concentrated to a combined level of about 20% in asmall liquid residue so that their collection and disposition arerelatively simple and inexpensive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flowsheet of a typical nuclear power plant-boilingwater reactor system illustrating one source of the gas mixtureseparated by this invention and a prior art system for removal ofradioactive components.

FIG. 2 is a schematic flowsheet of apparatus according to one embodimentof this invention for removing radioactive krypton from a nuclear powerplant-boiling water reactor off-gas mixture, as may be formed in theFIG. 1 system.

FIG. 3 is a schematic flowsheet on a larger scale of a three adsorbentbed prepurification system suitable for use in the FIG. 2 embodiment.

FIG. 4 is a preferred cycle and time program for the various steps ofthe FIG. 3 prepurification system.

FIG. 5 is a schematic flowsheet of another prepurification embodiment inwhich water is selectively removed in a first section and carbon dioxideselectively removed in a second separate section (not illustrated).

FIG. 6 is a schematic flowsheet of apparatus according to anotherembodiment in which krytpon and xenon are separately recovered as highpurity products from the nitrogen distillation column bottoms liquid.

FIG. 7 is a schematic flowsheet of a two adsorbent bed prepurificationsystem suitable for use in the FIG. 2 embodiment with minormodifications, and

FIG. 8 is a preferred cycle and time program for the various steps ofthe FIG. 7 prepurification system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 shows reactor 11, a large tank inwhich fissionable fuel element 12, as for example a tubular shellcontaining the fissionable material, is immersed in water. Small amountsof radioactive isotopes of Kr and Xe are produced as by-products of theatomic fission, and these gases collect inside the tubular shells.Inevitably, some of the gas will leak through imperfections in theshells into the boiling water and will be mixed with the steam deliveredby the reactor.

The steam generated in the reactor 12 at perhaps 500° F. is deliveredthrough conduit 13 to power-producing turbine 14 and the exhaust steamis discharged into vacuum condenser 15. There, the steam is condensed atabout 1 inch Hg total pressure by indirect heat exchange against waterentering at about 80° F. through conduit 16, and leaving at about 105°F. Condensate collected in condenser 15 is withdrawn through conduit 18and returned to the reactor 11 by means of pump 19.

The vacuum condenser 15 tends to be a collecting point for anynon-condensables in the steam cycle. In addition to the Kr and Xe, somewater is radiolytically decomposed in the reactor and the resultantoxygen and hydrogen are also carried by the steam into the condenserwhere they accumulate in the gas phase with Kr and Xe. Metallurgicalimperfections in the vacuum condenser 15 permit a significant amount ofair from the atmosphere to leak into the condenser and this air alsocollects with the foregoing gases. This mixture of non-condensable gasescomprises the off-gas produced by the reactor system.

FIG. 1 illustrates a prior art system for reducing the volume ofoff-gas, by removing a part of the non-radioactive and short-livedradioactive constituents, so that the long-lived radioactive fractioncan be held more economically in decay-storage until its totalradioactivity subsides to low levels. The gases accumulating incondenser 15 are removed and pumped to about 2 psig. pressure by meansof ejector 20 operated by high pressure stream 21. After the ejector,the resultant gas stream 22 consists primarily of steam and is preheatedin exchanger 23 by steam in conduit 24 and thereafter passed tocatalytic recombiner 25. By contact with a suitable catalyst material asfor example platinum or palladium impregnated on aluminum at about 900°F., oxygen and hydrogen components of the stream which were producedfrom water in the reactor 25 will thereby be recombined instoichiometric ratio to reform water. This step reduces significantlythe non-condensable fraction of the off-gas, e.g. by 75%. Aftercatalytic recombination, the stream is recooled in heat exchanger 26,where the water condenses and the stream is separated by vessel 27 intoa residual gaseous portion (off-gas) 28 containing the non-condensableradioactive components, and a condensate stream 29. The condensate ispumped back to the reactor 25 by means not shown. It is evident that thereduction in volume of the off-gas obtained in this manner is notsufficient to make extended storage feasible, and this expedient alsoignores the long-term problem created by Kr-85.

If the only non-condensables remaining in off-gas were Kr and Xe, thevolume of the stream would be exceedingly small. The disposition ofKr-85 and the solution to the long-term problem would be relativelysimple and inexpensive. However, as stated previously, the vacuumcondenser 15 inevitably contains minute leaks and a significant amountof air will enter the gases collecting in the condenser. The volume ofair thus added to the off-gas stream is essentially unaffected bypassage through catalytic reactor 25. The nitrogen, argon, krypton, andxenon components of the air leakage are chemically inert and do notreact. The oxygen introduced by air leakage does not react because thetotal hydrogen content of the gas is in stoichiometric balance with onlythat oxygen produced in the reactor 25 by radiolytic decomposition ofwater. Therefore, the air leakage into condenser 15 appears in itsentirety in off-gas 28 and its volume is on the order of one milliontimes that of the radioactive Kr-Xe which it contains. Thus, despite thereduction in off-gas volume obtained by O₂ --H₂ recombination, the airleakage is still responsible for a prohibitively large volume ofoff-gas, and this prevents the economic disposition of the smallradioactive Kr--Xe fraction.

The composition of the off-gas in conduit 28 includes radioactive Kr andXe (if present) from the reactor, normal components of air inleakage(N₂, O₂, argon hydrocarbons, CO₂, Kr and Xe) and moisture which escapedcondensation and removal in stream 29.

According to this invention, the Kr and Xe components are removed fromthe off-gas in a distillation column at cryogenic temperature, in whichthe gas is washed thoroughly with liquid nitrogen. The vent gas(primarily nitrogen) leaving the top of the column contains less than0.001% of the Kr and Xe entering the system, while the kettle liquid atthe bottom gradually increases in Kr--Xe content to a level of about 20mol %. The accumulation of Kr--Xe in the kettle is so slow that it ispossible to operate the system many weeks without necessarilywithdrawing product for disposal.

Processing the off-gas through a liquid nitrogen washing step presents aproblem due to the oxygen and hydrocarbon content of the gas. Thesecomponents tend to concentrate in the kettle of the wash column alongwith the Kr and Xe and thereby create an explosion hazard. In prior artcryogenic distillation systems, e.g. air separation, a somewhat similarhazard is eliminated by recirculating the kettle liquid through anadsorption trap to remove the hydrocabons and also carbon dioxide. Thisexpedient is not suited to the present system because the withdrawalrate of bottom product is so low that methane tends to accumulate, andan adsorption trap is not effective for removing this hydrocabon. Thepresence of oxygen in the feed (and hence in the kettle) creates afurther hazard due to the formation of ozone and nitrogen oxides in thehigh radioactive environment of the Kr-Xe containing kettle liquid.

In a preferred embodiment of this invention, the oxygen content of thefeed gas mixture is eliminated prior to its distillation by adding tothe off-gas from condenser 15, a quantity of hydrogen in excess ofstoichiometric ratio with the oxygen, and thereby converting most of theoxygen to water in recombiner 25. The hydrogen is added upstream of heatexchanger 23 and its rate of addition takes into account the hydrogenalready present due to radiolytic decomposition of water in the reactor.Thus the oxygen content of the feed is reduced to about 3000-4000 ppm incatalytic recombiner 25. Oxygen content in this range would permitaccumulation of oxygen in the kettle of the subsequent distillation stepto levels approaching 80% and would create a potential explosion hazarddue to the presence of methane and due to the potential formation ofozone and nitrogen oxides. Therefore, further reduction in oxygen toless than 10 ppm. and preferably to less than 1 ppm. is achieved instill another catalytic combustion unit in the feed stream prior todistillation. The foregoing low levels of oxygen are needed beforedistillation because the higher boiling components (higher thannitrogen) are concentrated in the distillation column by a factor on theorder of 3000. Thus, reducing the feed content to the distillationcolumn to 10 ppm. O₂ should limit O₂ content in the kettle liquid toabout 30,000 ppm. (3.0%). For preferred practice, oxygen is reduced inthe column feed to 1 ppm. and would limit O₂ content in the kettle to3000 ppm. (0.3%). Present-day technology actually permits substantiallymore complete oxygen removal by catalytic combustion. An oxygen level ofabout 0.1 ppm. is achievable after two-stage catalytic combustion and isa practical operating condition whenever a maximum factor of safety isdesired.

The CO₂ content of the off-gas (and any moisture present) also present aproblem because they freeze and foul the cold components of a cryogenicsystem. In the present invention, these constituents are removed fromthe feed gas mixture in a novel adsorption prepurified system. Afraction of the radioactive Kr is coadsorbed on the beds, and with aconventional pressure-swing or thermal-swing adsorption process thisco-adsorbed contaminant would be rejected to the atmosphere during thedesorption of CO₂ and H₂ O. The resultant discharge of radioactivematerial would be prohibitive. The problem is solved in this inventionby the use of a combination pressure-swing, thermal-swing system usingcrystalline zeolite molecular sieve adsorbent whereby the co-adsorbed Krcan be removed separately from the beds by low pressure cool purge priorto desorption of CO₂ and H₂ O. The separate stream containing thedesorbed Kr and Xe is recycled to the feed entering the prepurifier andthe Kr and Xe are ultimately accumulated with the major portion of theseelements in the kettle product of the distillation column. After Kr andXe desorption, the CO₂ and water are removed at higher temperature in anuncontaminated hot purge gas stream and are vented safely to theatmosphere. A three-bed parallel flow adsorption system is preferred asthe prepurifier component of the present invention.

As previously indicated, an excess of hydrogen is added for completeoxygen removal in catalytic recombiner 25. This unavoidably results inthe formation of methane in the recombiner, due to the presence of CO₂ :

    co.sub.2 + 4 h.sub.2 ⃡ ch.sub.4 + 2 h.sub.2 o  (1)

the additional methane accumulates to such levels in the bottom of thedistillation column that it not only creates the aforementioned hazardbut also tends to interfere with the effective separation of nitrogenfrom the Kr--Xe. If the temperature of the kettle were increasedsufficiently to drive the methane up the column and thereby suppress itsaccumulation in the kettle, then the risk would be greater that Kr willalso escape to the atmosphere in the overhead. Moreover, theuncontrolled accumulation of methane in the kettle would reduce the Krconcentration of the kettle liquid and would materially increase thevolume of Kr product sent to long-term storage.

In the present invention, methane and oxygen are held to low levels bymeans of a combustion system which may process the main feed stream offluid from the distillation column kettle. In the latter embodiment, afraction of the methane-enriched kettle liquid is withdrawn, vaporizedand heated, mixed as required with sufficient oxygen to react with themethane and passed through a catalytic combustion chamber. The resultantwater is removed by condensation and the remaining gas, also rich inradioactive Kr, is preferably recycled with the feed gas through thefinal stage of oxygen removal and through the prepurification system.Thus, its O₂, CO₂ and residual moisture content is removed beforechilling to cryogenic temperature. The foregoing bottom-product-recyclefeature not only reduces methane to a desire level in the kettle, butalso reduces O₂ in the kettle to only a few parts per million and farbelow a hazardous level.

The overhead gas from the distillation column will be essentially freeof all long-lived radioactive components, and that portion of the gasnot recondensed for column reflux may be vented to the atmosphere.Because the residence time of the off-gas in the system is only about2-5 minutes, the distillation column overhead gas will possess someresidual radioactivity due to short-lived isotopes. For example, theradioactive isotopes of nitrogen will possess short half-lives between0.1 and 10 minutes. Their release to the atmosphere is often notobjectionable. In those locations where such release cannot betolerated, another preferred embodiment of this invention provides asystem for the efficient storage of the nitrogen-rich overhead fluid fora sufficient time to obtain decay of its radioactivity level to anacceptable level. For this purpose, the entire quantity of thenitrogen-rich vapor is condensed in the top refluxing section of thedistillation column. A part of this nitrogen-rich condensate iswithdrawn and downwardly cascaded through a multiplicity of liquidretention zones for sufficient time delay for radioactive decay of theso-called activation gases, i.e. radioactive N₂, O₂ and argon. Among theisotopes the longest half-life is on the order of 10 minutes, andpreferably the total time provided in the liquid retention zones is atleast 30 minutes. The resulting deactivated liquid is returned to thedistillation column as part of the nitrogen liquid refrigerant used tocondense the nitrogen-rich vapor. As previously indicated, the nitrogenoverhead gas from the distillation column may ultimately be vented tothe atmosphere without appreciable radioactive contamination.

One embodiment of the system of this invention will be described indetail with reference to FIG. 2.

The numerical quantities contained in this description are illustrativeonly and are based upon a 1100 megawatt power plant discharging anoff-gas from the vacuum condenser 11 at a maximum rate of about 275standard cubic feet per minute. The off-gas with a radioactivity levelof about 200 × 10⁶ μ Ci/sec. is compressed from the vacuum condenser toabout 5 psig. in stream ejector 20 (not illustrated in FIG. 2). At thispoint, externally supplied hydrogen gas is introduced and mixed with theoff-gas through conduit 30 at a rate controlled by valve 31, which inturn is regulated by flow ratio controller 32 in response to a signalreceived from analyzer 33. Analyzer 33 monitors the oxygen content ofthe off-gas exclusive of the oxygen which is in stoichiometric ratiowith the hydrogen present, and generates a signal to flow ratiocontroller 32 for appropriate adjustment of flow controller 34 and valve31. The admission of hydrogen through valve 31 is in excess of thestoichiometric ratio with the oxygen measured by analyzer 33 by apredetermined factor. The value of the factor is set by downstreamanalyzer 35, which monitors the excess of hydrogen persisting in theoff-gas.

It will be understood that after such hydrogen introduction, thehydrogen content of the feed gas mixture is slightly in excess ofstoichiometric ratio with the entire oxygen content rather than withonly that portion of the oxygen produced radiolytically in the reactor11. The gas mixture is about 92.7% steam and is processed through heater23 and first catalytic recombiner 25, where oxygen and hydrogen arereacted to form water. The steam content is needed in this step tocontrol the temperature in the first recombiner 25 and to dilute theoxygen-hydrogen content below the explosive limit.

After the recombiner, the gas mixture is cooled in exchanger 26 againstcooling water and the condensate is removed in separator 27. The removalof the steam results in a mixture composed primarily of nitrogen (95.7%by volume) with residual hydrogen (4.0%) and oxygen (0.3%). Two recyclestreams are now added to the feed gas from origins to be describedlater. These are the distillation column bottoms product reacted recyclegas in conduit 36 (comprising primarily N₂ and argon)), and the Kr/Xe -containing nitrogen purge gas in conduit 37 from the prepurifiersection. At this point in the system, perhaps 2 minutes time lapse willhave occurred since the gas left the condenser 11 and its radioactivitywill have decayed substantially, for example to about 2.17 × 10⁶ μCi/sec. in the case of the illustrative 1100 megawatt power plant. Theaddition of the aforementioned recycle streams increases theradioactivity level only by about 10,000 μ Ci/sec. However, asignificant fraction of the radioactivity of the recycle streams isproduced by long-lived isotopes and their removal is important despitetheir relatively low level of total radioactivity. The combined gasmixture is pressurized in compressor 38 to about 100 psig., reheated(e.g. electrically or by steam) in passageway 39 to 200° F., andintroduced to second catalytic combustion chamber 41 for contact withcatalyst material, for example platinum or palladium impregnatedalumina, for further reduction of its oxygen content. Themoisture-containing gas is then recooled in exchanger 42 and condensateis removed in separator 43 through conduit 44. Two catalytic recombiners25 and 41 are provided in series flow relationship, the first unit beingoperated at intermediate temperature as for example 900° F. and contacttime of about 2 seconds, the high steam content being used to controlthe temperature at this level and to suppress the concentration ofcombustible components. With this mode of operation, the oxygen may notbe completely recombined so that the gas mixture downstream separator 27still contains 0.3 mol. % O₂. The second catalytic recombiner 41 servesto reduce this oxygen concentration to a level suitable for processingin the cryogenic portion of the system, e.g. below 1.0 ppm. Recombiner41 is operated at a relatively low intermediate temperature of belowabout 400° F. so as to minimize the undesirable formation of methane inaccordance with equation (1). However, it is contemplated that a singlecatalytic combiner might be used instead of the two units. In the FIG. 2embodiment this may be accomplished by conducting the catalytic reactionstep at higher temperature, e.g. 1400° F., and with adequate contacttime the oxygen content of the feed gas may be reduced to 1.0 ppm. orless. In this instance, heater 39 and second catalytic recombiner 41 areeliminated. However, with one catalytic recombiner the high temperaturerequires more expensive materials of construction, and the large volumeof the feed stream (due to its water content) if retained in the unitfor requisite contact time, requires a relatively large reactor.Moreover, the maximum safe upper limits of oxygen and hydrogen of 2% and4% respectively will not produce sufficient heat of combustion to raisethe stream temperatutre to 1400° F., and additional energy must beintroduced to preheat the stream.

The moisture and CO₂ are removed in prepurifier section 46 comprising atleast two and preferably three crystalline zeolite molecular sieve beds47, 48 and 49 suitably manifolded in parallel flow relation foralternate, sequential operation. In these beds, moisture and CO₂ arepreferentially adsorbed by the crystalline zeolitic molecular sieve,e.g. synthetic materials such as calcium zeolite A (5A), described inMilton U.S. Pat. No. 2,882,243 and sodium zeolite X (13X) described inMilton U.S. Pat. No. 2,887,244. Naturally occurring crystalline zeolitessuch as chabazite and mordenite may alternatively be used. Zeoliticmolecular sieves also selectively adsorb krypton and xenon, but not asstrongly as moisture and CO₂, so that a minor part of the former arecoadsorbed in prepurification 46. As illustrated, the gas mixture atsuperatmospheric pressure and ambient temperature is passed throughfirst zone or bed 47 while second bed 48 is being countercurrentlypurged of krypton and xenon adsorbate by cool nitrogen gas at aboutambient temperature and low pressure slightly above atmospheric. At thesame time, third adsorbent bed 49 is countercurrently purged of waterand CO₂ adsorbate by flowing hot nitrogen purge gas therethrough at lowpressure for discharge from the feed inlet end into conduit 50 andeventual release to the atmosphere.

The prepurified gas mixture (free of water and CO₂) discharged fromfirst bed 47 flows through conduit 51 and throttling valve 52 where itspressure is reduced to approximately 28 psig. It is then cooled tocryogenic temperature, e.g. -304° F. in passageway 53 of heat exchanger54 by nitrogen overhead gas in passageway 55. The cryogenically cooledprepurified gas mixture is introduced to distillation column 56 at anintermediate level below rectifying section 57 and above strippingsection 58 each comprising a series of superimposed liquid-gas contacttrays. Distillation column 56 also includes top reflux condenser 59above rectifying section 57 and bottom kettle 60 with heating means 61as for example an electric coil.

Liquid nitrogen refrigerant stored in container 62 is introduced throughconduit 63 and control valve 63a to the outer jacket of top refluxcondenser 59 at suitable low pressure such that it boils while coolingand condensing at least part of the krypton-depleted vapor introduced topassageways 64 through conduit 65 joining the upper end of rectifyingsection 57. The at least partially condensed krypton-depleted mixture isflowed from passageways 64 to vessel 65a for phase separation and atleast part of the condensate returned through conduit 66 as reflux tothe rectifying section upper end. This liquid flows downwardly in massand heat exchange with rising cryogenically cooled prepurified gas andkrypton partially depleted vapor at relatively low liquid to vaporvolume ratio to wash out the Kr., e.g. 0.27, and form the aforementionedKr-depleted vapor and Kr-enriched liquid. High L/V ratios would providestill more complete separation but at added cost of condensing moreliquid reflux. Reducing the L/V value appreciably below 0.27 would lendto Kr leakage out the top of the column.

The uncondensed portion of the fluid emerging from reflux condenserpassageways 64 and entering vessel 65a comprises hydrogen anduncondensed nitrogen. This vapor is recirculated through conduit 66a tothe off-gas feed stream entering the system in conduit 13, The junctureis upstream the point at which the oxygen concentration is determined byanalyzer 33 to control the hydrogen introduction through conduit 30. Therecovered hydrogen in conduit 66a accounts for about 10% of the total H₂needed for O₂ removal. Stripping section 58 receives thekrypton-enriched liquid from the rectifying section lower end, theliquid passing downwardly in mass and heat exchange with rising kettlevapor at relatively high liquid to vapor mol ratio for total boilup,e.g. 1.0, to form Kr partially depleted vapor and kettle liquid. Thewithdrawal rate of krypton-xenon bottoms concentrate liquid throughconduit 67 by control valve 68 is extremely low relative to the refluxrate in stripping section 58 so that the column preferably operates withessentially total boilup. Heat for boiling the kettle liquid is byelectric heater 61, as previously indicated. All of the krypton andxenon content of the feed gas, including both radioactive andnonradioactive fractions thereof, are washed from this gas mixture bythe liquid nitrogen and accumulate in kettle 60. With total boilup andessentially zero loss of kryton-xenon in the overhead, they graduallyand progressively accumulate and eventually reach a concentration ofabout 20% in kettle 60 after one to two years operation. This product isperiodically withdrawn through conduit 67, vaporized in passageway 69 bywarmer fluid is passageway 70 and pressurized in compressor 71 forholding in cylinders 72. The latter may for example be stored inshielded vaults for a period of time such as one year, sufficient forits radioactivity to decay to a level safe for final disposition or use.

Returning now to top reflux condenser 59, the N₂ refrigerant liquidintroduced thereto through conduit 63 comprises the externally suppliedportion from storage container 62 and the nitrogen condensate fromseparator 65 not required for return through conduit 66 to thedistillation column as reflux. Any such nitrogen condensate flowsthrough branch conduit 73 and hold-up column 74 (discussed hereinafter)to conduit 75 and pressure reducing control valve 76 therein for joiningwith the externally supplied liquid nitrogen in conduit 63.

In one mode of operating distillation column 56, only the fraction ofKr-depleted vapor in conduit 65 is liquefied in passageways 64 as neededfor reflux in rectifying section 57 and all refrigerant liquid issupplied from storage container 62. In this operating mode, there is noliquid flow through conduit 73 and after pressure reduction in valve66b, the unliquefied Kr-depleted vapor fraction from separator 65apasses through conduit 77 (dotted portion) to join vaporized nitrogenoverhead gas in conduit 78 from reflux condenser 59.

In another mode of operating distillation column 56, the overheadKr-depleted vapor in conduit 65 is substantially totally liquefied andthe condensate collected in separator 65a is divided with one portionbeing returned to the column through conduit 66 and the remainder beingpassed through delay column 74 and pressure reduction valve 76 to thejacket of reflux condenser 59, as previously described. The uncondensedfraction 66a from separator 65a recirculates to feed conduit 13 upstreamof the addition of hydrogen from source 30 so as to recover the slightexcess of hydrogen originally introduced for catalytic combustion. Ifdesired, stream 66a may be rewarmed in a separate passage of heatexchanger 54 prior to rejoining feed conduit 13. When delay column 74 isemployed, it is desirable to operate the distillation column 56 at anelevated pressure on the order of 50 psig. so as to achieve morecomplete condensation of nitrogen from the vent stream 66a and therebyreduce the nitrogen recirculated through the system.

In order to suppress the accumulation of methane (resulting fromequation (1)) in kettle 60 below levels detrimental to the completeremoval of krypton from the column overhead product, a portion of thekettle liquid withdrawn through conduit 67 and control valve 68 isdirected through branch conduit 36. This portion is vaporized, e.g. byatmospheric heat in passageway 80 and a metered quantity of air isintroduced to the vapor stream through conduit 81 and control valve 82therein. The quantity of air contains at least sufficient oxygen toreact stoichiometrically with the methane content of the vapor inaccordance with the following equation:

    CH.sub.4 + 2 0.sub.2 → CO.sub.2 + H.sub.2 O         (2)

preferably a substantial excess of air is added such that the methanecontent is diluted to about 1%. This is well below the flammabilitylimit of methane and in addition it avoids excessive temperature in thesubsequent catalytic combustion step. The mixture is then passed tocatalytic combustion chamber 83 filled with an alumina-supportedplatinum electrically heated by element 84, and the reaction product isreturned to the system upstream the impurity removal steps. Asillustrated, the reaction product in conduit 36 is returned to the feedgas conduit 13 at the inlet to compressor 38 for reprocessing. Thus, itsresidual oxygen content is removed by recirculation through secondcatalytic combustion chamber 41 and its CO₂ and residual moisturecontent is eliminated in prepurifier section 46. Alternatively, thereaction product may be purified of CO₂, water and O₂ in a separatesystem and returned directly to the distillation column kettle 60. Suchseparate system might include an H₂ -- O₂ recombiner for residual O₂, adrier and a CO₂ adsorption zone.

The withdrawal and recycle of a fraction of the kettle liquid not onlysuppresses the methane level in the column but also reduces the oxygenlevel for example to about 0.03% when oxygen in the column feed is 0.1ppm. Since the lower explosion limit of oxygen in about 6% methane isseveral percent, it is clear that any explosion hazard due to oxygen iseliminated by a comfortable margin. The low level of oxygen achieved inthe kettle also eliminates hazards due to the formation of ozone andoxides of nitrogen.

Flow of the feed gas mixture (off-gas) through the second catalyticrecombiner 41, prepurifier 46 and distillation column 56 requires only afew (e.g. less than ten) minutes, so that the vapor discharged asdistillation column purified nitrogen overhead gas in conduit 78 stillpossesses a relatively high radioactivity level, e.g. about 125,000microcurries per second for the aforementioned 1100 megawatt plant. Thisradioactive content is due primarily to so-called activation gases, i.e.isotopes of nitrogen, oxygen and argon whose half-lives are less than 10minutes. Since the radioactive components exhibiting long half-lives areessentially completely removed from the vapor in the column, theoverhead column vapor can often be safely discharged to the atmospherewithout danger of accumulative contamination of the environment. Theoverhead gas in conduit 78 is partially rewarmed to about -28° F. inpassageway 55 for the aforedescribed cooling of the feed gas tocryogenic temperature. The partially rewarmed nitrogen overhead gasdischarged from heat exchanger 54 is further rewarmed to about 70°-104°F. in passageway 85 as for example by atmospheric or steam heat, andthereafter employed to purge and regenerate the adsorbent beds ofprepurification section 46.

If the short term radioactivity of the nitrogen overhead gas from thedistillation column precludes direct venting, another preferredembodiment of this invention provides an effective system for delayingthe flow of the liquefied Kr-depleted vapor from the reflux condenser64. This delay is for a time sufficient for the radioactivity to decayto acceptable low levels preferably at least 30 minutes. Subsequentlythe time-delayed Kr-depleted liquid nitrogen may be used as part of theneeded refrigerant in the reflux condenser and the resulting nitrogenvapor vented to the atmosphere after purging water and CO₂ fromprepurification section 46. A preferred time delay system comprisescolumn 74 containing a multiplicity of superimposed and verticallyspaced baffles or trays arranged to permit a slow progressive cascade ofthe liquid from tray-to-tray down the column. By way of illustration, inone design twenty trays are sufficient to contain the liquid for 175minutes. After this delay period the radioactivity of the so-calledactivation gases (oxygen, nitrogen, argon) will have decayed to a verylow level. For the 1100 megawatt plant the total residual radioactivitywill be perhaps 130-140 microcurries/second. A large part of thisresidual radioactivity is contributed by various relatively short-livedisotopes, and the vaporized liquid is safe for venting to theatmosphere.

As previously indicated, the radioactive krypton/xenon adsorbate isremoved by purge gas at ambient temperature and at low pressure beforethe beds are regenerated, i.e. cleaned of CO₂ and water by hot, lowpressure purge gas, at for example 600° F. To accomplish this, therewarmed nitrogen gas in conduit 78 downstream heat exchanger 85 isdivided into a first minor part in branch conduit 86 and a second majorpart in branch conduit 87. The first minor part, comprising for exampleabout 20% by volume of the feed gas flow to the prepurifier section 46,is directed to the discharge end of second adsorbent bed 48 for flowtherethrough in a direction countercurrent to the previously flowingfeed gas. It should be noted that prior to this ambient temperaturepurging step and on completion of its feed gas selective adsorption step(preferably terminated when the carbon dioxide adsorption front isintermediate the inlet and discharge ends, i.e. partial loading, toavoid CO₂ breakthrough in the prepurified gas), second bed 48 iscountercurrently depressurized to slightly above atmospheric pressure byrelease of gas from its feed inlet end through conduit 37 and controlvalve 88. This stream comprises void space gas including radioactivekrypton/xenon, and therefore should not be released to the atmosphere.It is returned to the feed gas conduit 13 upstream compressor 38 forfurther processing in the previously described manner.

Returning to the first and ambient temperature purge step in second bed48, the cool gas is introduced at low pressure slightly above ambientpressure, e.g. 2 psig, and the countercurrent purging of krypton/xenonadsorbate continues for a time period corresponding to most of the firstbed adsorption step, e.g. 6 hours of a total 8 hour period. Thecoadsorbed krypton and xenon are substantially completely removed fromsecond bed 48 during this time, and carried in conduit 37 to the inletof compressor 38 for reprocessing. During this purge step a smallportion, e.g. about 10% of the CO₂ adsorbate and about 5% of H₂ Oadsorbate using zeolite 13X, will also be unavoidably desorbed duringthis purging. The purge gas flows countercurrent to the previouslyflowing feed gas to insure complete removal of water from the adsobentbed in the succeeding hot purge step.

The second major part of nitrogen purge gas in conduit 87 is furtherwarmed in passageway 89 by heater 90 to at least 400° F. and for exampleabout 600° F., and directed at low pressure to the discharge end ofthird molecular sieve adsorbent bed 49 for flow therethrough in adirection countercurrent to the previously flowing feed gas mixture.This hot gas serves to desorb the strongly held water and CO₂ andregenerate the bed for subsequent processing of feed gas mixture. Flowis countercurrent to insure that any water adsorbate is substantiallycompletely removed, as residual water would significantly reduce the CO₂loading in the subsequent adsorption step. The water and carbon dioxidedischarged to the atmosphere through conduit 50.

The operation of prepurifier section 46 will be described in greaterdetail in connection with FIGS. 3 and 4, but in brief the threemolecular sieve adsorbent beds 47, 48 and 49 are connected in parallelflow relation with appropriate valving for cyclic operation andcontinuous processing of the feed gas mixture.

Referring to FIGS. 3 and 4, elements corresponding to previouslydescribed elements in FIGS. 1 and 2 have been assigned the sameidentification numerals to facilitate comparison. The feed gas mixturein conduit 13 after having been pressurized in compressor 38 andprocessed through second catalytic combustion chamber 41, cooler 42 andphase separator 43 enters the adsorption system through any of feedvalves 91a, 91b or 91c. The prepurified gas is discharged from theadsorption system through any of product valves 92a, 92b or 92c intoconduit 51. The nitrogen overhead gas from the distillation column,having been reduced in pressure and rewarmed in heat exchangers 54 and85, enters the adsorption system through conduit 78a. The cool firstminor part thereof in conduit 86 (the ambient temperature portion) isdirected through any of valves 93a, 93b or 93c to the feed discharge endof the adsorption system. The resulting krypton/xenon containing purgegas leaves the system through any of valves 94a, 94b or 94c at the feedinlet end, and is returned by conduit 37 to the suction side ofcompressor 38.

The second part of the nitrogen gas in conduit 87 is warmed to about600° F. by electrical heater 90 and the hot gas flows through any ofvalves 95a, 95b or 95c at the feed discharge end of the adsorbent bedsfrom which krypton and xenon have just been removed. This hot, lowpressure purge gas desorbs the more strongly held CO₂ and wateradsorbate, and is vented to the atmosphere through any of valves 96a,96b or 96c and conduit 50 at the feed inlet end.

FIG. 4 shows that an adsorbent bed may remain on the adsorption step foreight hours or one-third of a complete cycle. After the adsorption step,the bed is first depressurized countercurrently ("depress.") from thefeed gas level, e.g. 80 psig., to atmospheric pressure. Next, the bed ispurged for six hours with substantially ambient temperature nitrogen("cool purge") to the end of the fifteenth hour. Thereafter the "hotpurge" gas is introduced and the bed is heated for a period of fourhours to remove CO₂ and water (to the end of the nineteenth hour). Nowthe bed is recooled in about four hours by continuing the flow of theunheated third part of nitrogen purge gas through bypass conduit 95around heater 90 and any of valves 95a, 95b or 95c at the feed dischargeend and the corresponding valve 96a, 96b or 96c at the feed inlet end ofsystem. A final period of sixty minutes is used for repressurizing therecooled bed to the feed gas pressure, e.g. from atmospheric to 80psig., by admitting feed gas through any of valves 91a, 91b or 91c withall valves which join the discharge end of the adsorbent bed beingclosed so that the latter is "dead-ended." The flow control valves91-95a, b and c are preferably automatically operated in accordance witha predetermined time cycle.

The horizontal lines in FIG. 4 show the flow relationships between thethree adsorbent beds at any point of time in the respective cycles. Forexample, during the 7th-8th hour of the first bed adsorption step, feedgas is also introduced to the second bed inlet end for repressurization("repress") from atmospheric pressure to the feed gas pressure -- thelast step of the cycle. During the 8th-9th hour the horizontal line fromthe first bed depressurization indicates this gas rejoins the feed (bymeans of conduit 37 in FIGS. 2 and 3). The four vertical lines representthe manifolds for the major fluid streams, which are joined by thehorizontal lines to the individual beds. Reading from left to right, thevertical lines represent the feed gas conduit 13, the vent gas conduit50, the purified nitrogen overhead gas conduit 78a from the distillationcolumn, and the prepurified gas conduit 51 joining the "cryo-unit." Thelatter block includes heat exchanger 54, distillation column 56, liquidnitrogen storage tank 62, and time delay column 74.

In the aforedescribed embodiment of the prepurifier section 46, each ofthe three adsorbent beds is used to selectively coadsorb both carbondioxide and water (if present) from the feed gas mixture using the samecrystalline zeolite molecular sieve adsorbent material. Alternatively,the prepurifier section may comprise a first zone for selectivelyadsorbing water, and a second zone for the selective adsorption ofcarbon dioxide from the H₂ O -- free feed gas mixture plus theunavoidable coadsorption of a small quantity of krypton, and xenon (ifpresent). One potential advantage is separating the two sections is thata relatively inexpensive adsorbent such as alumina may be used to removewater, thereby extending the duration of the feed gas processing cyclestep for the more expensive molecular sieve material and/or possiblypermitting the use of smaller beds for CO₂ adsorption. Another potentialadvantage is that the CO₂ -removing molecular sieve section may beregenerated at lower temperature, with consequent savings in materialsand operating costs. Finally, separating water adsorption from CO₂adsorption may materially reduce the flow of cool purge gas and hencereduce the recycle of gas through the system.

Referring now more specifically to FIG. 5, only a portion of theprepurification system and components directly related thereto, areshown. Other nonrelated components may be substantially identical tothose already described and illustrated in FIG. 2 (or the succeedingFIG. 6). The feed gas mixture in conduit 13 has been freed of most orall of its oxygen content, and supplimented by the distillation columnkettle recycle stream in conduit 36 (after methane-oxygen reaction toproduce carbon dioxide and water) and by depressurization gas and coolnitrogen purge gas from the prepurification system in conduit 37(contaning krypton desorbate). This gas mixture is pressurized forexample to about 80 psig. in compressor 38, the compression heat removedin cooler 42 and condensed water removed in separator 43.

The resulting gas mixture in conduit 101 is directed to one bed of atwo-bed alumina drier section 102 for residual water removal. Forexample the gas may flow through inlet valve 103, first alumina bed 104and discharge valve 105 to effluent conduit 106. The residual moisturecontent of the feed gas is thereby reduced to a dewpoint not greaterthan -60° F. When first bed 104 is loaded with water, the alternatesecond alumina bed 107 is placed "on stream" and bed 104 is regeneratedpreferably by countercurrent purging at elevated temperature.

The regeneration gas may for example comprise dried process gascirculated through conduit 108 by blower 109 and control valve 110 toheater 111 where its temperature is increased to 350°-650° F. Theresulting hot gas still in conduit 108 is directed by branch conduit120, regeneration manifold 113 and valve 114a to the feed gas dischargeend of first alumina bed 104. The water-laden regeneration gas iswithdrawn through valve 115b in regeneration gas manifold 116 at thefeed gas inlet end, and passed through joining conduit 108 to cooler117. The condensed water is removed in separator 118 and thewater-depleted gas is recirculated through blower 109.

This closed loop recirculation of hot regeneration gas continues untilthe water is substantially removed from the alumina bed 104. The lattermay now be returned to the normal temperature (about ambient) forprocessing feed gas, by circulating the same regeneration gas throughbed 104 without being heated and in the direction cocurrent to the feedgas. For this purpose, valve 110 in conduit 108 and valve 111a areclosed to respectively remove the water separator 118 and heater 111from the circuit. Valve 119 in conduit 120 and valve 121 in conduit 122are open, thereby establishing a cooling gas flow circuit opposite tothe hot regeneration gas circuit. Gas from blower 109 is divertedthrough conduit 122 and valve 121 therein, and cooler 117 in conduit 108to manifold 116. The cooling gas thus flows through valve 115a into thefeed gas inlet end of bed 104 and continues through valve 114a indischarge end manifold 113, conduit 120 and valve 119, back to blower109. Such recirculation of cooldown gas continues, until the bed isagain restored to its operative temperature, such as 100° F.

Normally, the use of water cooler 117 will be sufficient to dry the bedand obtain the desired -60° F. dewpoint or below. However, when moderateregeneration temperatures are employed it may be desirable to extractwater more completely from the closed loop regeneration gas to morepositively insure the required low moisture content in the prepurifiedfeed gas. To this end, more thorough water removal can be attained bythe use of an additional cooling passageway 123. The latter is in heatexchange relationship with at least a part of the gas flowing inpassageway 85 of FIG. 2 which is thereafter directed through passageway124.

When a drier bed is being regenerated and the regenerating gas withinthe closed loop is heated to elevated temperature, the gas will expandand the pressure in the loop will rise. Such pressure rise is notdetrimental provided the system is designed to withstand the higherpressure. However, if uniform pressure is preferred throughout theentire operating cycle of the driers, then the excess pressure duringregeneration can be bled off from the system. For this purpose, conduit125 is branched from conduit 122 and gas from which water has beenremoved is released from the loop through valve 126 and is returned tothe process feed gas conduit 101 for continued treatment in the driersection 102 of the prepurification system. Valve 126 is a backpressureregulator which opens and permits flow therethrough until pressures aresubstantially equalized thereacross. Upon completion of regeneration,the gas in the loop will now contract during the subsequent coolingstroke and the pressure will tend to drop. The gas which was vented fromthe loop during regeneration is now replenished to restore the pressureby bleeding dried feed gas from dried gas effluent manifold 127 throughcheck valve 128 into the regeneration gas loop.

The moisture-depleted feed gas in conduit 106 may be directed to a threebed system 46 as described in connection with FIG. 2, but only servingto remove carbon dioxide. The apparatus, process, and cycle steps areidentical to those employed for both CO₂ and water removal except thatthe purge gas may be treated to moderate elevated temperature for onlyCO₂ desorption, e.g. 350° instead of 400° F. and above required forwater removal.

Whereas embodiments of the prepurifier system described hereinabove haveemployed three adsorption beds, it should be understood that a systemcomprising only two beds is a useful alternative. With reference to FIG.7 and its associated cycle sequence FIG. 8 using the same 1100 megawattpower plant example, the partially-treated feed gas stream having passedthrough compresser 38 and second catalytic combustion chamber (FIG. 2)41 is introduced to the prepurifier section through conduit 45. Tofacilitate comparison in these figures and the ensuing description,items identical to those previously described in the three bedembodiment of FIG. 3 are assigned the same numeral. Adsorbent beds 200and 201 are connected in parallel by appropriate manifolds at both feedand product ends thereof. With first bed 200 in the selective adsorptionstep, the feed stream passes through valve 202 into bed 200 andprepurified gas is discharged through valve 204 and conduit 51 to thelow temperature heat exchanger 54. While first bed 200 is on theselective adsorption step, alternate or second bed 201 proceedssequentially through each of the previously described steps ofdepressurization, cool purge, hot purge, recooling and repressurization.For the depressurization step, all valves associated with second bed 201are closed except valve 209 at the inlet end. Gas countercurrentlyreleased from second bed 201 passes through valve 209, trim valve 213 inconduit 219 to recycle conduit 37. As shown in FIG. 2, conduit 37returns to the inlet of compresser 38 so that the desorbed krypton andxenon in the depressurization gas can be effectively recovered in theon-stream bed. Depressurization requires about one-tenth hour and thepressure of the bed is reduced from 80 psig. to about 1 atmosphere.

The cool purge step is conducted by directing the purified nitrogenoverhead gas (conduit 78b) at reduced pressure from the cryogenic unitwhich has been rewarmed in heat exchanger passages 55 and 85 throughheater bypass valve 96 and conduit 95 to the product end of second bed201. The gas passes through valve 207, bed 201, valve 209, valve 214 inconduit 218 and thence to recycle conduit 37. For this step, trim valve213 employed during depressurization to prevent undesirably high gasvelocities in second bed 201 is closed and purge valve 214 is opened tominimize flow restriction in the purge circuit. The cool purge steprequires about 6.9 hours and is conducted at substantially ambienttemperature.

The hot purge step is conducted by directing the purified nitrogen gasfrom the cryogenic system at low pressure through heater passage 89,valve 207, bed 201, and valve 211 to vent 50. Valves 209 and 214 areclosed for this step. The heat front passing through the bed raises theadsorbent temperature to at least 300° F. and the hot purge flow iscontinued for about 3.5 hours. If moisture is being removed in beds 200,201 the absorbent temperature should be raised to at least 350° F.

The recooling step is conducted in the same manner as the hot purgestep, except that the purified nitrogen gas bypasses heater 89 throughvalve 96 and conduit 95. The flow of cool purge gas continues for about3.4 hours and sweeps the heat front out the feed end of the bed therebyrecooling the adsorbent to near ambient temperature.

The repressurization of bed 201 is accomplished with feed gas asdescribed in connection with FIG. 2 and such repressurization gas passesthrough conduit 216, valves 215 and 209 to the feed inlet of the bed.Valves 211 and 207 are closed. The bed is thus repressurized from aboutone atmosphere pressure to about 80 psig. in 0.1 hour. Whenrepressurization is complete, second bed 201 is placed on-stream byclosing valves 209 and 215 and opening valves 203 and 205.Simultaneously, first bed 200 is removed from the selective adsorptionstep by closing valves 202 and 204. The latter bed is then advancedthrough all the foregoing steps described for the purge and regenerationof second bed 201.

With only two beds in the system, the recycle flow of cold purge gasback to the feed gas conduit is intermittent rather than continuous andspecial components are provided in FIG. 7 to smooth and equalize theflow of total feed to the prepurifier. This is accomplished by means ofpurified gas bypass conduit 227 containing control valve 224. In effect,this conduit recirculates purified nitrogen through the portion of theprocess comprising the prepurifier and the cryogenic purifier. Therecirculation is controlled to compensate for the flow or absence offlow of cool purge gas through this portion of the process. When coolpurge gas is being recirculated through conduit 218 to recycle conduit37, then flow through the bypass conduit 227 is discontinued, but whenthe flow of cool purge is terminated (i.e., during the hot purge step),then flow through bypass conduit 227 is regulated so as to circulate anequivalent amount of purified nitrogen from the cryogenic column 56 tothe feed stream in conduit 28 and thereby maintain uniform flow in thesystem.

The control of flow through equalizing conduit 227 may be accomplishedby sensing the rate of flow through prepurified gas conduit 51 and bysensing the rate of flow through recycle conduit 37 by which therecirculation gas is returned to the suction side of compressor 38 (FIG.2). The point at which flow in recycle conduit 37 is sensed, i.e. atorifice 220, is downstream of the juncture of cool purge conduit 218 andpurified nitrogen bypass conduit 227, and the signal obtained at 220 isused to maintain the flow constant despite changes in flow through coolpurge conduit 218. The electric or pneumatic signal is directed to flowindicator controller 222 which in turn generates a signal 228 forregulating valve 224 located in bypass conduit 227. A signal fromorifice 221 in prepurified gas conduit 51 is used to adjust the setpointof controller 222 such that a constant ratio is maintained between therecycle gas flow in conduit 37 and the prepurified gas flow in conduit51. Means (not illustrated) are provided for generating signalsrepresentative of the differential sensed across orifices 220 and 221,which signals are transmitted to controller 222.

During such regulation of bypass gas in conduit 227, valve 212 inconduit 95 which supplies hot purge to regenerate an adsorbent bed 200or 201 is maintained full open. The full open position of valve 212 isobtained by means of an automatic on-off valve 225 located in thepneumatic signal transmission means 228 to valve 212 and is preferablyoperated by the timer control which initiates the changes in theadsorbent bed sequencing valves. Valve 225 serves to vent the pressurein the pneumatic signal means 228 to valve 212, and upon suchoccurrence, valve 212 assumes a full open position.

When the hot purge step is not in progress and cool purge gas is insteadbeing circulated through the prepurifier section, valve 225 isrepositioned to restore pneumatic signal pressure through 228 to valve212 and place it under the control of controller 222. Simultaneously,automatic timer controlled valve 230 vents the pneumatic pressure signalfrom transmission means 228 to bypass control valve 224 which isdesigned so that the effect of such venting is to close the valve. Thisterminates the flow of bypass nitrogen.

Summarizing the FIGS. 7-8 two-adsorption zone prepurifier embodimentcool, low pressure purge gas and hot purge gas are consecutively passedthrough the second adsorption zone during a portion of the period thefeed gas mixture is passing through the first adsorption zone. Duringthe hot purge gas flow an amount of partially rewarmed purified nitrogenoverhead gas equivalent to the cool purge gas flow is joined with thefeed gas mixture for passing through the first zone. The following stepsare sequentially conducted in each of the two adsorption zones:introducing feed gas mixture, releasing gas from the zone fromsuperatmospheric pressure to about atmospheric pressure, removing thekrypton coadsorbate by the first part of cool nitrogen purge gas,removing the carbon dioxide adsorbate by the further warmed second partof hot nitrogen purge gas, recooling the cleaned zone using a third partof the partially rewarmed purified nitrogen overhead gas, andrepressurizing the recooled cleaned zone by the feed gas mixture.

This invention may also be used to separate radioactive krypton fromoff-gas derived from sources other than boiling water reactor-typenuclear power plants, as for example off-gas from nuclear fuelprocessing systems. The off-gas from fuel reprocessing differs insignificant respects from that released by the previously describedboiling water reactor system. Spent fuel rods arrive at the reprocessingplant in special shielded casks. Considerable time will have elapsedsince removal of the rods from the reactor so that short-lived isotopestrapped within the cladding will have decayed to a relatively lowradioactive level. In fact, the radioactivity of the xenon component ofthe trapped gases will normally be less than 1/10000 of theradioactivity of the krypton component.

The spent rods are first sheared apart and then leached in nitric acidto dissolve the heavy metal fuel elements. The rod structure containsconsiderable material such as zirconia and carbon which does notdissolve, and which is separated from the liquor and discarded. Theliquor is further processed to recover the fuel value therein.

Gaseous products entrapped in the rods are released during shearing andleaching and constitute the major part of the radioactive material inthe off-gas. In addition, gaseous nitrogen oxides (NO and NO₂) areproduced from the nitric acid during leaching and these compounds areincluded in the off-gas. Gaseous hydrocarbons are also produced byreaction of carbon and steam and will become part of the off-gas. Air isalso present due to leakage. A typical composition of nuclear fuelreprocessing off-gas is as follows (dry basis):

No and NO₂ -- 6%

Hydrocarbons (as CH₄) -- 1000 ppm.

Kr -- 200 ppm.

Xe -- 2000 ppm.

Air -- Balance

In the off-gas purification system, the Kr at minimum will be removed.Xe will also be removed because, once Kr has been washed from theoff-gas, the Xe, which possesses lower volatility, will also becompletely washed out as a matter of course. In the embodiment to bedescribed, Kr and Xe will be recovered separately rather than as a mixedproduct, the concentrated Kr being sent to long-term decay storage andthe Xe to short-term storage as required for essentially completedeactivation. Not more than 90 days storage will normally be needed forcomplete deactivation of the Xe, after which it may be used commerciallyor disposed of as desired.

The separation of Kr by cryogenic distillation is not feasible with theamounts of hydrocarbons and nitrogen oxides contained in the feed. Thevolatilities and boiling points of these components are such as tointerfere grossly with the desired separation. Hence, both hydrocarbonsand nitrogen oxides are converted catalytically prior to distillation.In addition, oxygen is removed by combustion with hydrogen as in theboiling water reactor off-gas purification system.

In this embodiment, the krypton-xenon concentrate liquid from thepreviously described first distillation is itself distilled to produce axenon bottom concentrate liquid and further enriched krypton overheadgas. This xenon product may be stored and subsequently used after itsradioactivity has dissipated, and the krypton gas may be separatelystored for radioactive decay. The previously described purified nitrogenoverhead gas-prepurified feed gas mixture heat exchange is dividedbetween a colder zone and a warmer zone, with the partially rewarmednitrogen overhead gas from the colder zone being heat exchanged with thefurther enriched krypton overhead gas from the second distillation forpartial condensation thereof. The resulting further rewarmed nitrogengas is thereafter heat exchanged with the prepurified feed gas in theaforementioned warmer zone. A major part of the uncondensed furtherenriched krypton gas from the partial condensation of the seconddistillation overhead gas is joined with the feed gas mixture prior tothe prepurification step and the remaining minor part of the furtherenriched krypton gas is discharged as product.

As previously indicated, the feed gas mixture contains hydrocarbons,nitrogen oxides and oxygen impurities. Prior to the prepurificationthese impurities are removed by the steps of first catalyticallyreacting the hydrocarbons with part of the oxygen to form water andcarbon dioxide, and adding diverted impurity-depleted gas mixture to thehydrocarbon-depleted gas mixture to form an augmented gas mixture. Astoichiometric excess of hydrogen is added to this augmented mixture andthe oxygen and nitrogen oxides content thereof is reacted with thishydrogen to form water and nitrogen in a second catalytic reaction stepto form the impurity-depleted gas mixture. The latter is cooled and atleast 85% by volume (dry basis) and preferably at least 90% is diverted.The diverted impurity-depleted gas mixture is heat exchanged with theimpurity-depleted gas mixture for partial cooling of the latter andrewarming of the diverted gas. The rewarmed gas comprises the divertedimpurity-depleted gas which is added to the hydrocarbon-depleted gasmixture. The undiverted cooled impurity-depleted gas mixture is passedto the prepurification.

In addition to the previously described apparatus this embodimentincludes a second distillation column having a multiplicity of spacedliquid-gas contact trays, a bottom kettle with heating means, and asecond top reflux condenser. Conduit means are provided for transferringkrypton-xenon bottom concentrate liquid from the bottom kettle of thefirst distillation column to an intermediate level of the seconddistillation column for separation into xenon bottom concentrate liquidand further emriched krypton overhead gas.

The heat exchange means having a first passageway for the prepurifiedgas and the second passageway for the nitrogen overhead gas comprises acolder part and a warmer part with conduit means for flowing partiallyrewarmed nitrogen overhead gas from the colder part to the second topreflux condenser of the second distillation column for cooling andpartially condensing further enriched krypton gas. Other conduit meansare provided for flowing further rewarmed nitrogen overhead gas from thesecond top reflux condenser to the cold end of the second passageway ofthe heat exchange means warmer part. First conduit means are includedfor joining a major part of the uncondensed further enriched krypton gasfrom the second top reflux condenser with the feed gas mixture. Thisapparatus embodiment also includes further enriched krypton gas storagemeans and second conduit means for passing the remaining minor part ofthe uncondensed further enriched krypton gas to the storage means.

With reference to FIG. 6, a feed gas mixture of the above typicalcomposition, enters the system in conduit 13 and joins a Kr recyclestream in conduit 177. The recycle stream whose purpose will beexplained hereinafter, is small in volume but rich in Kr so that the Krcontent of the feed gas mixture is increased about tenfold to about 2000ppm. The combined stream is heated to 800° F. in electrical preheater 23and passes to first catalytic converter 150 containing an oxidizingcatalyst for example platinum or palladium supported on alumina, wherehydrocarbons are burned with oxygen in the feed, producing water andCO₂. About 95% of the hydrocarbons are converted leaving about 50 ppm.unconverted CH₄ in the gas.

The hydrocarbon-depleted stream leaves first converter 150 and joins adiverted impurity depleted feed gas stream which has been freed ofoxygen and depleted in nitrogen oxides. This diverted stream in conduit15 and flow regulated by valve 152, is about tenfold greater in volumethan the incoming feed gas and has two purposes; (a) it dilutes theoxygen content below about 2 volume % so that upon subsequent admissionof hydrogen, the mixture will be below the explosive limit, and (b) itserves as a means of control of the downstream second catalyticrecombiner 25 wherein it avoids excessive temperature otherwise causedby the strong exothermic reactions. The augmented stream is joined by astream of hydrogen from conduit 30 which is introduced in meteredquantity regulated by automatic valve 31, responsive to the incomingfeed flow rate for example sensed by an orifice and also to the oxygenand nitrogen oxide content of the gas sensed by analyzer 33. The amountof hydrogen introduced is sufficient to produce a ratio of hydrogen to(oxygen + nitrogen oxides) in slight excess of the stoichiometric ratio.

The mixed stream passes to second catalytic recombiner 25 where theoxygen content is combined with hydrogen to form water, and the nitrogenoxides combine with hydrogen to produce water and nitrogen. Theresultant stream is about 1400° F. as a consequence of the reactions,and is sufficiently hot so that the formation of additional CH₄ by themethanation reaction is insignificant. Such high temperature isfavorable to the reduction of nitrogen oxides, and also to the virtualcomplete removal of free oxygen by reaction with hydrogen. The stream ispartially cooled in passageway 153 of heat exchanger 154 by the divertedimpurity-depleted gas mixture in passageway 155, further cooled againstcooling water in exchanger 26 and is separated from condensed water inseparator 27. If desired, the water-depleted stream may then be passedthrough a third catalytic recombiner 41, containing a catalyst similarto that of recombiner 25, and any residual oxygen is further andpositively reduced to 0.1 ppm. or lower by combination with hydrogen.The impurity-depleted stream is now pressurized slightly in steamejector 40 and is divided, a controlled major portion such as 90% byvolume being diverted in conduit 151 and returned to the feed gas streamafter first hydrocarbon converter 150. The remaining minor portion ofthe impurity-depleted gas mixture in conduit 45 is conducted to theprepurification and further steps in the process.

The undiverted impurity-depleted gas mixture in conduit 45 is furtheraugmented by addition of depressurization gas and cold purge gas inconduit 37 from the prepurification, these streams being recycled torecover their Kr and Xe content. The joined streams are now compressedto about 80 psig. in compressor 38 and condensed water is removed inseparator 43. Then the remaining gas is conducted to one of threeprepurifier adsorbent beds 47, 48 or 49 where residual water and CO₂ areremoved (i.e. the adsorbate undergoing mass transfer during a completeadsorption-desorption cycle). The operation of the prepurifier sectionis substantially the same as previously described for the boiling waterrector off-gas prepurifier. The prepurified gas stream, now at about 75psig., may for example have the following approximate composition:

Nitrogen oxides -- 200 ppm.

Ch₄ -- 50 ppm.

Xe -- 2000 ppm.

Kr -- 2000 ppm.

N₂ -- balance

The gas is now ready for final cryogenic purification. It is chilled tocryogenic temperature in two steps, passageway 33 of warmer precoolingexchanger 54 and then in passageway 157 of colder precooling heatexchanger 158 before entering an intermediate level of first stagedistillation column 56. Column 56 separates the Kr and Xe into a mixedbottom concentrate liquid product and delivers a purified gaseousoverhead product which is substantially nitrogen and which ishereinafter preferred to as "nitrogen". The Kr depleted vapor leavesrectifying section 56 through conduit 65, passes through refluxcondenser 64 and the partially condensed stream is phase separated invessel 65a. The Kr-depleted condensate fraction is returned to thecolumn as reflux through conduit 66 while the gaseous fraction iswithdrawn through conduit 66a as nitrogen.

As stated previously, in this embodiment the Xe and Kr are produced asseparate products and this separation is accomplished in second stagedistillation column 165. The mixed Kr--Xe bottom product of first stagecolumn 56 is withdrawn through conduit 67 and introduced at anintermediate level of second stage 165 for separation into a very pureXe bottom concentrate liquid product and a further enriched Kr gaseousoverhead product. Heat is supplied to the kettle 166, for exampleelectrically by heaters 167. A major fraction of the further enrichedkrypton overhead gas emerging from the rectification section of column165 in conduit 168 is reliquefied in passageway 169 of condenser 170,separated from the vapor in vessel 171 and the liquid returned to therectification section through conduit 172, thereby providing a highliquid to vapor molar ratio L/V in the upper section of the column ofabout 0.9. This is desirable to insure a high degree of Kr-Xeseparation. The uncondensed vapor fraction from separator 171 in conduit173 is warmed in passageway 174 by heat exchange, e.g. with the ambientatmosphere. A minor part, e.g. about 1/10 by volume of the furtherenriched Kr gas in conduit 173 is withdrawn as product through branchconduit 174a and is pressurized in compressor 175 for long-term storagein cylinders 176. The major part of warmed krypton in conduit 173comprises the Kr recycle stream and is joined through conduit 177 withthe incoming feed gas mixture in conduit 13.

One reason for recirculating a major part of the further enriched Krproduct back through the system is to obtain additional enrichment ofthe Kr product. With recirculation, the Kr content of the feed gas tofirst stage column 56 is about 2000 ppm. (dry basis) and the Kr producthas the typical analysis:

Kr -- 75%

Xe -- 5%

Ch₄ -- 5%

nitrogen oxides (NO) -- 15%

it will be apparent from the foregoing that the Kr enrichment factor forthe two columns 56 and 165 is greater than 350. However, withoutrecirculation, the Kr content of the prepurified feed gas to first stagecolumn 56 would be about 1/10 as high or about 200 ppm. With such feed,the Kr product would be limited in purity to 50% or less. Thus, asubstantially greater volume of gas would necessarily be sent to longterm decay storage in cylinder 176. Moreover, the Kr product wouldcontain considerably more Xe which in this embodiment is desired as aseparate, commercially-valuable product.

Another reason for recirculating Kr rich product is to control and limitthe buildup of CH₄, O₂ and nitrogen oxides in the kettle liquid of firststage column 56. With recirculation, the rate of withdrawal of Kr--Xebottom product from first stage column 56 is sufficiently high toprevent accumulation of such troublesome components. In second stagecolumn 165, the temperature of the kettle 166 is sufficiently high sothat such components are driven up the column through liquid-gas contacttrays and are withdrawn in the overhead gas. Upon recirculation, theCH₄, O₂ and nitrogen oxide components of the Kr recycle stream inconduit 177 are again subjected to catalytic conversion in units 150, 25and 41.

The bottom product from second column 165 is essentially pure Xe and, asstated previously, the radioactive isotopes thereof will usually havedecayed to a harmless level. Accordingly, the product may be furtherrefined as required for commercial use, or otherwise disposed.Momentary, short term decay storage may be practiced as a positive meansof insuring inactivity of the Xe product. The liquid product stream iswithdrawn from the bottom of second column 165 through conduit 178,vaporized in passageway 179 by heat exchange, e.g. with ambient air, andpressurized in compresser 181 for storage in cylinders 182.

The uncondensed vapor of the nitrogen overhead gas from first column 56in conduit 66a is pressure reduced in valve 66b and mixed with thevaporized nitrogen refrigerant system in conduit 78 from refluxcondenser 59 to form a combined purified nitrogen overhead gas stream inconduit 78a which is partially warmed to about 120 K. in passageway 183of colder precooling heat exchanger 158 against partially precolledprepurified feed gas enroute to first column 56. The resulting partiallywarmed nitrogen overhead gas in conduit 78a is above the freezingtemperature of Kr and flows through passageway 184 of second refluxcondenser 170 to cool and partially liquefy the further enriched Kroverhead gas in passageway 169 from the rectification section of secondcolumn 165. The further warmed nitrogen overhead gas continues inconduit 78a to passageway 55 of warmer precooler heat exchanger 54 whereit is still further warmed to about ambient temperature againstprepurified feed gas enroute to first column 56. The nitrogen gasemerging from the warm end of warmer exchanger 54 is now processed inthe same manner as described in connection with the FIG. 2 system.

Although preferred embodiments of this invention have been described indetail, it will be appreciated that other embodiments are contemplatedalong with modifications of the disclosed features, as being within thescope of the invention.

What is claimed is:
 1. Apparatus for the separation of radioactivekrypton from a feed gas mixture comprising nitrogen and trace amounts ofcarbon dioxide and said radioactive krypton comprising;a. at least twocrystalline zeolite molecular sieve adsorbent beds arranged in parallelflow relation; b. means for providing said feed gas mixture atsuperatmospheric pressure and ambient temperature and sequentiallyintroducing same to the inlet end of each of said adsorbent beds; c.heat exchange means having first and second passageways and means forpassing nonadsorbed prepurified gas from the discharge end of each ofsaid adsorbent beds to said first passageway for cryogenic coolingtherein; d. a distillation column comprising a top reflux condenser, abottom kettle with heating means, and a multiplicity of spacedliquid-gas contact trays intermediate said top reflux condenser and saidbottom kettle; e. a liquid nitrogen supply and means for introducingsame to the top reflux condenser; f. conduit means for introducing thecryogenically cooled prepurified gas from said heat exchanger means toan intermediate tray section of said distillation column for mass andheat exchange with krypton-depleted condensate to form krypton-depletedvapor and krypton-enriched liquid; g. a conduit means for passing saidkrypton-depleted vapor from the upper end of the distillation columnintermediate tray section to said top reflux condenser for heat exchangewith said liquid nitrogen supply to form nitrogen overhead gas andkrypton-depleted condensate, and means for returning at least part ofsaid krypton-depleted condensate to the upper end of said intermediatetray section; h. conduit means for passing said nitrogen overhead gas tosaid second passageway of said heat exchanger means for partiallyrewarming same to about ambient temperature and for said cryogeniccooling of said prepurified gas; i. other conduit and flow control meansfor sequentially passing a first part of the partially rewarmed nitrogenoverhead gas at low pressure as cool purge gas to the feed discharge endof each of said adsorbent beds having previously been at least partiallyloaded with carbon dioxide and krypton from said feed gas mixture, forsubstantially complete desorption of only said krypton; j. conduit meansfor returning the krypton-containing first part of cool purge gas fromthe adsorbent bed inlet end to the feed gas mixture providing means (b);k. means for further warming a second part of said partially rewarmednitrogen overhead gas as hot purge gas, and still other conduit and flowcontrol means for sequentially passing same to the feed discharge end ofeach of said adsorbent beds having previously been at least partiallyloaded with carbon dioxide and krypton from said gas mixture andthereafter substantially completely desorbed of only said krypton, fordesorption of the remaining carbon dioxide; l. conduit means fordischarging the carbon dioxide-containing hot purge gas from theadsorbent bed feed inlet end; and m. conduit and flow control means forsequentially introducing a third part of said partially rewarmednitrogen overhead gas to the feed discharge end of each of saidadsorbent beds after the hot purge gas flow for recooling thereof. 2.Apparatus according to claim 1 for the separation of radioactive kryptonand xenon from a feed gas mixture comprising nitrogen and trace amountsof carbon dioxide and said radioactive krypton and xenon, including asecond distillation column having a multiplicity of spaced liquid-gascontact trays, a bottom kettle with heating means, and a second topreflux condenser; and conduit means for transferring krypton-xenonbottom concentrate liquid from said bottom kettle of distillation column(d) to an intermediate level of said second distillation column forseparation into xenon bottom concentrate liquid and further enrichedkrypton overhead gas.
 3. Apparatus according to claim 2 with firstconduit means for joining a major part of said further enriched kryptonoverhead gas with the feed gas mixture providing means (b), furtherenriched krypton gas storage means, and second conduit means for passingthe remaining minor part and further enriched krypton gas thereto. 4.Apparatus according to claim 2 wherein heat exchanger means (c)comprises a colder part and a warmer part; conduit means for flowingpartially rewarmed nitrogen overhead gas from the second passageway ofthe heat exchanger means (c) colder part to said second top refluxcondenser for cooling and partially condensing further enriched kyrptongas, and conduit means for flowing further rewarmed nitrogen overheadgas from said second top reflux condenser to the cold end of the secondpassageway of the heat exchange means (c) warmer part; first conduitmeans for joining a major part of the uncondensed further enrichedkrypton gas with said feed gas mixture; further enriched krypton gasstorage and second conduit means for passing the remaining minor part ofsaid uncondensed further enriched krypton gas to said storage means. 5.Apparatus for the removal of hydrocarbons, nitrogen oxide and oxygenimpurities and separation of radioactive krypton and xenon from a feedgas mixture comprising nitrogen and trace amounts of carbondioxide, saidhydrocarbons, nitrogen oxide and oxygen impurities and said radioactivekrypton and xenon comprising:a. means for first catalytically reactingsaid hydrocarbons and part of said oxygen in said feed gas mixture toform water and carbon dioxide and thereby provide a hydrocarbon-depletedfeed gas mixture; b. means for sensing the oxygen concentration in saidhydrocarbon-depleted feed gas mixture; c. means for introducing hydrogento said hydrocarbon-depleted feed gas mixture in response to the oxygensensing means such that said feed gas mixture has a stoichiometricexcess of hydrogen; d. means for second catalytically reacting theoxygen and nitrogen oxide with said hydrogen in said feed gas mixture toproduce water; e. means for cooling the second catalytically reactedfeed gas mixture and separating the water to form a remaining feed gasmixture depleted in hydrocarbons, nitrogen oxide and oxygen impurities;f. at least two crystalline zeolite molecular sieve adsorbent bedsarranged in parallel flow relation; g. means for providing saidremaining feed gas mixture of (e) at superatmospheric pressure andambient temperature and sequentially introducing same to the inlet endof each of said adsorbent beds; h. heat exchange means having first andsecond passageways and means for passing nonadsorbed prepurified gasfrom the discharge end of each of said adsorbent beds to said firstpassageway for cryogenic cooling therein; i. a distillation columncomprising a top reflux condenser, a bottom kettle with heating means,and a multiplicity of spaced liquid-gas contact trays intermediate saidtop reflux condenser and said bottom kettle; j. a liquid nitrogen supplyand means for introducing same to the top reflux condenser; k. conduitmeans for introducing the cryogenically cooled prepurified gas from saidheat exchanger means to an intermediate tray section of saiddistillation column for mass and heat exchange with krypton-depletedcondensate to form krypton-depleted vapor and krypton-enriched liquid;l. conduit means for passing said krypton-depleted vapor from the upperend of the distillation column intermediate tray section to said topreflux condenser for heat exchange with said liquid nitrogen supply toform nitrogen overhead gas and krypton-depleted condensate, and meansfor returning at least part of said krypton-depleted condensate to theupper end of said intermediate tray section; m. conduit means forpassing said nitrogen overhead gas to said second passageway of saidheat exchanger means for partially rewarming same to about ambienttemperature and for said cryogenic cooling of said prepurified gas; n.other conduit and flow control means for sequentially passing a firstpart of the partially rewarmed nitrogen overhead gas at low pressure ascool purge gas to the feed discharge end of each of said adsorbent bedshaving previously been at least partially loaded with carbon dioxide andkrypton from said feed gas mixture, for substantially completedesorption of only said kyrpton; o. conduit means for returning thekrypton-containing first part of cool purge gas from the adsorbent bedinlet end to the feed gas mixture providing means (g); p. means forfurther warming a second part of said partially rewarmed nitrogenoverhead gas as hot purge gas, and still other conduit and flow controlmeans for sequentially passing same to the feed discharge end of each ofsaid adsorbent beds having previously been at least partially loadedwith carbon dioxide and krypton from said gas mixture and thereaftersubstantially completely desorbed of only said krypton, for desorptionof the remaining carbon dioxide; q. conduit means for discharging thecarbon dioxide-containing hot purge gas from the adsorbent bed feedinlet end; r. conduit and flow control means for sequentiallyintroducing a third part of said partially rewarmed nitrogen overheadgas to the feed discharge end of each of said adsorbent beds after thehot purge gas flow for recooling thereof; s. a second distillationcolumn having a multiplicity of spaced liquid-gas contact trays, abottom kettle with heating means and a second to reflux condenser; andt. conduit means for transferring krypton-xenon bottom concentrateliquid from said bottom kettle of distillation column (i) to anintermediate level of said second distillation column for separationinto xenon bottom concentrate liquid and further enriched kryptonover-head gas.
 6. Apparatus according to claim 5 with means diverting amajor part of said remaining feed gas mixture from said means forpassing same to means (g), means for heat exchanging the diverted majorpart with second catalytically reacted feed gas mixture as part of thecooling thereof, and means for returning such diverted major part to thefeed gas mixture supply means upstream the oxygen concentration sensingmeans and downstream the first catalytic reacting means.